As an alternative to rubber, the aluminum Billet Traktor Knob Set ($80) will set you back a few bucks, but it will add a classic retro appearance to your unit that will come in handy for the long run.  You can buy a set from Tempo Records on the west coast, but I am not sure if you can mix and match colors (they do come in a few colors), so if you wish to add some property value to your investment, look no further.